I use Opera 11.50.14 on almost bare maemo. That webmail works when I check only tls1.2 in opera:config. When I check all tls 1.0, 1.1 and 1.2 this page fails with 552 code.

So try type opera:config into url field and into search type 'tls' and uncheck 1.0 and 1.1 and check only tls 1.2. Should be good on opera 12 also.